Detection of serum antibody to Brucella abortus in cattle by use of a quantitative fluorometric immunoassay.

A quantitative fluorometric immunoassay (FIAX) was adapted for the detection of serum antibodies to Brucella abortus in cattle. Results are expressed in nanograms of immunoglobulin binding the antigen carrier. The FIAX was compared with the standard tube agglutination, Rivanol precipitation, and complement fixation tests, using 285 serum samples from vaccinated, challenged, or control cattle. Linear regression analysis indicated a significant correlation among all four serological tests; the FIAX test correlated best with the Rivanol test. Ninety sera were from vaccinated and nonvaccinated cattle that were challenged with virulent B. abortus 2308. The sensitivity and specificity of each serological test were determined based on culture results from these cattle. The FIAX was the most sensitive of the four serological tests, detecting 79.2% of the culture-positive animals. The FIAX was the least specific, with 15.4% of the culture-negative animals being classified as positive. Eighty-eight sera were from cattle vaccinated with strain 19 but not challenged. All four serological tests had a statistically significant ability to distinguish sera from control and vaccinates on the basis of mean titers. The mean titer of vaccinates was also significantly different from that of challenged animals. Advantages and disadvantages of the FIAX test for bovine brucellosis are discussed.